1. Field of the Invention
The present invention relates to a transformer, which can ensure insulation and a sufficient creepage distance between a primary and a secondary winding.
2. Description of the Related Art
Sheet transformers comprise a core and primary and secondary printed wiring.
As shown in FIG. 10, the core comprises a pair of E-type core parts 51 and 52 which are formed in an xe2x80x9cExe2x80x9d shape in their side view. The E-type core parts are combined together so as to form the core. The E-type core parts 51 and 52 include cylindrical core legs 51a and 52a, respectively, in the center thereof.
As shown in FIG. 11, the primary winding is formed on a substrate 53. The substrate 53 has a hole 53a through which the core legs 51a and 52a of the E-type core parts 51 and 52 penetrate and a winding which is printed on a surface of the printed wiring board 53.
This printed wiring board 53 is incorporated with the core legs 51a and 52a of the E-type core parts 51 and 52, thereby forming a sheet transformer. A part of the magnetic path is formed by the core legs 51a and 52a. 
For transformers, there are a variety of safety standards of various countries. In accordance with the requirements of the standards, it is necessary to insulate between the primary winding (input coil) and the secondary winding (output coil).
Among international safety standard for transformers, IEC (International Electro-technical Commission) 950 is for office supplies, etc. IEC 950 restricts the thickness of an insulating material arranged between the primary and second windings and the creepage distance or a spatial distance between the primary and secondary windings, in accordance with the used voltage.
In the as described above, where the primary winding and the secondary winding are formed on different printed wiring boards, it is possible to ensure a predetermined thickness of an insulating paper between the printed wiring boards, but there is a difficult to ensure a predetermined creepage distance between the primary and secondary windings.
Thus, it is difficult to satisfy the safety standard for transformers. Therefore, the thin-type transformer can be used only for low-voltage power source, such as a DC-DC converter equal to or lower than 48V, etc. that does not seriously require the insulation.
The present invention has been made in consideration of the above. It is accordingly an object of the present invention to provide a transformer which has the structure for ensuring the insulation and the creepage distance between a first and second windings.
Another object thereof is to provide a transformer which safely operates with high reliability.
In order to attain the above objects, according to the first aspect of the present invention, there is provided a transformer comprising:
a first substrate which includes a first winding;
a second substrate which includes a second winding and an opening in center of the second winding;
an insulating material which covers the first substrate and insulates between the first winding and the second winding; and
a core which has two ends holding the first substrate covered with the insulating material through the insulating material therebetween, and which forms a part of the magnetic path passing through the first winding and the second winding.
According to such a structure of the transformer, it is possible to ensure the insulation and a creepage distance between the first winding and the second winding.
The first substrate may include an opening in a center portion of the first winding and a core member arranged in the opening of the first substrate;
the insulating material may cover the first substrate including the core member; and
the core may be so formed that the core member of the first substrate is sandwiched between the two ends through the insulating material.
The core and the core member may be to form a part of the magnetic path which passes through the first winding and the second winding.
The first substrate may include an opening in a center portion of the first winding; and
the core may be so formed that the opening of the first substrate is sandwiched between the two ends through the insulating material.
The opening may form a gap of the magnetic path passing through the first winding and the second winding.
The core may include a first core leg and a second cored leg respectively forming a part of the magnetic path;
the first substrate may be sandwiched between the first and second core legs through the insulating material; and
one of the first and second core legs may penetrate through the opening formed in said second winding of the second substrate.
Each of the first and second core legs may penetrate through the opening formed in said second winding of the second substrate.
Each of the first winding and the second winding may be formed from a sheet coil, and both of the first and second windings may be stacked up one after another;
the insulating material may comprise a sheet-like insulating material which covers over the first winding; and
the first core leg and second core leg of the core may be so arranged that center of the first winding covered with the insulating material is sandwiched therebetween.
The second winding may comprise two sheet coils between which the first winding is arranged; and
the transformer further may comprise a second insulating material which covers over at least one of the sheet coils constituting the second winding.
In order to attain the above objects, according to the second aspect of the present invention, there is provided a transformer comprising:
a first sheet coil;
a second sheet coil which is stacked on the first sheet coil and has mutual induction with the first sheet coil;
an insulating material which so covers over the first sheet coil as to insulate first and second sheet coils from each other and ensure a creepage distance between the first and second sheet coils; and
a core which is so formed that the insulating material covering over the first sheet coil is sandwiched between the two ends thereof and which is to form a magnetic patch through the first and second sheet coil.
The core may be formed of ferrite.
A core member may be arranged in a center portion of the first sheet coil; and
the core member and the core may be to form a magnetic path passing through center portions of the first and second sheet coils.
The center portion of the first sheet coil may have an opening; and
the opening may serve as a gap of a part of the magnetic path formed by the core.